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HOW DOES A COMPUTER PROCESS INFORMATION? and What are the key differences between a diskette and a hard drive? | download free pdf
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MANAGING PUBLIC SECTOR RECORDS A Training Programme Understanding Computers: An Overview for Records and Archives Staff INTERNATIONAL INTERNATIONAL RECORDS COUNCIL ON ARCHIVES MANAGEMENT TRUST WHAT IS A COMPUTER? A computer allows users to store and process information quickly and automatically. A computer is a programmable machine. It allows the user to store all sorts of information and then ‘process’ that information, or data, or carry out actions with the information, such as calculating numbers or organising words. Computer: A machine that can receive and store information and change or process it. Information: Knowledge that is communicated. Data (pl.): The representation of information in a formalised manner suitable for communication, interpretation and processing, generally by a computer system. Note: the term ‘raw data’ refers to unprocessed information. Computers can be generally classified by size and power, although there can be considerable overlap. Following are descriptions of several different types of computers. Mainframe computers are large-sized, powerful multi-user computers that can support concurrent programs. That means, they can perform different actions or ‘processes’ at the same time. Mainframe computers can be used by as many as hundreds or thousands of users at the same time. Large organisations may use a mainframe computer to execute large-scale processes such as processing the organisation’s payroll. Mini-computers are mid-sized multi-processing computers. Again, they can perform several actions at the same time and can support from 4 to 200 users simultaneously. In recent years the distinction between mini-computers and small mainframes has become blurred. Often the distinction depends upon how the manufacturer wants to market its machines. Organisations may use a mini-computer for such tasks as managing the information in a small financial system or maintaining a small database of information about registrations or applications. Workstations are powerful, single-user computers. They have the capacity to store and process large quantities of data, but they are only used by one person at a time. However, workstations are typically linked together to form a computer network called a local area network, which means that several people, such as staff in an office, can communicate with each other and share electronic files and data. UNDERSTANDING COMPUTERS 5 Computer network: A grouping of computers and peripherals connected together by telecommunications links to enable a group of users to share and exchange information. Networks are discussed in more detail later in this module. A workstation is similar to a personal computer but is more powerful and often comes with a higher-quality monitor. In terms of computing power, workstations lie in between personal computers and mini-computers. Workstations commonly support applications that require relatively high-quality graphics capabilities and a lot of memory, such as desktop publishing, software development and engineering applications. Personal computers (PCs), also called microcomputers, are the most popular type of computer in use today. The PC is a small-sized, relatively inexpensive computer designed for an individual user. Today, the world of PCs is basically divided between IBM-compatible and Macintosh-compatible machines, named after the two computer manufacturers. Computers may be called ‘desktop’ computers, which stay on the desk, or ‘laptop’ computers, which are lightweight and portable. Organisations and individuals use PCs for a wide range of tasks, including word processing, accounting, desktop publishing, preparation and delivery of presentations, organisation of spreadsheets and database management. Entry-level PCs are much more powerful than a few years ago, and today there is little distinction between PCs and workstations. Activity 1 If your office has computers, find out the type or types. Are they mainframe computers, mini-computers, workstations or personal computers? What ‘processes’ or actions are the computers used for? Write a brief description of the types of computers in place and their main uses. If your office has more than one type of computer, find out why? What different tasks are the different computers intended to do? If your office does not have computers, try to contact a colleague or friend who has a computer and ask him or her what type he or she has and what primary functions it is used for. UNDERSTANDING COMPUTERS 6 WHAT ARE THE COMPONENTS OF A COMPUTER? Computers are made up of two parts: the hardware and the software. Hardware: The physical equipment required to create, use, manipulate and store electronic data. Software: The computerised instructions that operate a computer, manipulate the data and execute particular functions or tasks. All computers require the following hardware components: • central processing unit (CPU) Central processing unit (CPU): The chip or chips at the heart of a computer that enable it to process data. Also known as a processor. • memory Memory: An area within a computer system that holds data waiting to be processed. • storage device Storage device: The place where a computer puts data. • input devices : the devices that allow data and instructions to enter a computer (such as a keyboard, mouse, scanner) Input: Any resource required for the functioning of a process, in the course of which it will be transformed into one or more outputs. • output devices: the devices that allow information to be represented (that is, given out) to the user, such as a display screen or printer) Output: The product of the transformation of inputs by a process. UNDERSTANDING COMPUTERS 7 Printers, scanners and external disk drives that may be connected to the computer are also sometimes called ‘peripheral devices’. Peripheral device: Any piece of equipment in a computer system that is not actually inside the computer itself. The central processing unit (CPU) is the heart of the computer. It carries out all of the instructions given in a program, such as a word processing or spreadsheet program. The CPU consists of one or more chips (another name for “integrated circuits”). Chip: A small piece of semi-conducting material (such as silicon) about 1 centimetre (¼ inch) square on which an integrated circuit is embedded. An integrated circuit is a number of electronic components joined together to form a path for electricity. Central processing unit chips contain the circuits representing the CPU. A microprocessor is a particular type of chip. The original IBM personal computer used the Intel 8088 microprocessor. Most of today’s microcomputers are designed around a microprocessor from one of two product families: x86 or Power. The 80286, 80386, and 80486 models that followed were referred to by the last three digits, 286, 386, and 486. For the next generation, however, Intel broke with tradition and introduced the Pentium in 1993. In 1997, it introduced the Pentium II to address multi-media applications, and most recently the Pentium III to address the new opportunities provided by access to large volumes of information on the world wide Web. Other manufacturers of chips (such as Cyrix) produce chips of similar power and capabilities. CPU’s are not all equal. Some process data faster than others. A computer contains a system clock that emits pulses to establish the timing of all systems operations. The system clock operates at a speed quite different from a clock that keeps track of the time of the day. The system clock determines the speed at which the computer can execute an instruction, and therefore limits the number of instructions the computer can complete within a specific amount of time. The time to complete an instruction execution cycle is measured in megahertz (MHz) or millions of cycles per second. Although some instructions require multiple cycles to complete, the processor speed should be thought of in terms of the number of instructions the processor can execute in one second. Today, microprocessor speeds exceed 300 MHz. If all other specifications are identical, then higher megahertz ratings means faster processing. When determining what type of computer you are using or considering what type of computer to acquire, it is important to know that these terms – 286, 386, 486, Pentium – refer to the type of processor in the computer. Newer computers will come with Pentium microprocessors (or the equivalent from other manufacturers); older ones with microprocessors from the x86 family. UNDERSTANDING COMPUTERS 8 It is important to know the type of processor in your computer. Some newer computer programs will not run on older processors, and some newer processors are too sophisticated for older software. The faster the processor in a computer, the more quickly the computer will perform operations. The most common type of memory that most users are familiar with is ‘main memory’ or ‘RAM’ (random-access memory). Random access memory (RAM): An area in the computer system unit that temporarily holds a user’s data, operating system instructions and program instructions. The word ‘main’ is used to distinguish it from external mass storage devices such as the hard drive or disk drives. Note that the term ‘mass storage’ refers to various techniques and devices for storing large amounts of data; mass storage is distinct from memory because it retains data even when the computer is turned off. Thus mass storage is sometimes referred to as ‘auxiliary storage’. Following are definitions of common storage devices: Storage: The area within a computer system where data can be left on a longer term basis while it is not needed for processing. Diskette. A small, removable, flexible mylar plastic disk covered with a thin layer of a magnetisable substance, onto which digital data can be recorded and stored. Also known as a floppy disk. Hard drive: The storage area within the computer itself, where megabytes of space are available to store bits of information. Also known as a hard disk. Optical disk: A storage device that uses reflecting surfaces and laser technology to read and write data on a disk. Also known as a laser disk. Magnetic tape: A continuous plastic strip covered with magnetic oxide; the tape is divided into parallel tracks onto which data may be recorded by selectively magnetising parts of the surface, or spots, in each of the tracks. The data can then be stored and reused. UNDERSTANDING COMPUTERS 9 Activity 2 If your office has a microcomputer, look at the manuals that come with it. Review the manuals to find out what type of microprocessor is in the computer. Look also at the computer screen when the computer is turned on to see if information about the computer is shown there. Then examine two types of software used by your organisation. What are the minimum requirements for a microprocessor for each type of software? Does the computer meet the minimum requirements for the software? Does it exceed the requirements (that is, is the computer’s microprocessor newer or more powerful than the software requires)? Again, look in the software manuals for information, or ask a friend or colleague to help you. Do not remove the back from the computer or try to unload or reload the software. HOW DOES A COMPUTER WORK? A computer functions in the following manner: • The computer accepts input. Computer input is whatever is entered or fed into a computer system. Input can be supplied by a person (such as by using a keyboard) or by another computer or device (such as a diskette or CD-ROM). Some examples of input include the words and symbols in a document, numbers for a calculation, instructions for completing a process, pictures, and so on. • The computer performs useful operations, manipulating the data in many ways. This manipulation is called processing. Examples of processing include performing calculations, sorting lists of words or numbers, modifying documents and pictures according to user instructions, and drawing graphs. A computer processes data in the CPU. Process: A systematic series of actions a computer uses to manipulate data. • The computer stores data. A computer must store data so that it is available for processing. Most computers have more than one location for storing data (the hard drive or C:\, and the floppy drive or A:\). The place where the computer stores the data depends on how the data is being used. The computer puts the data in one place while it is waiting to be processed and another place when it is not needed for immediate processing. The storage of data in the computer is called ‘online storage’ while the storage of data on computer tapes, diskettes or CD-ROMs is called ‘offline storage’. UNDERSTANDING COMPUTERS 10 • The computer produces output. Computer output is information that has been produced by a computer. Some examples of computer output include reports, documents, music, graphs, and pictures. Output can be in several different formats, such as paper, diskette, or on screen. A computer receives data as input, processes it, stores it and then produces output. Activity 3 Describe, in terms of input, processing and data, the production of a letter using a computer. What happens first? What happens last? HOW DOES THE SOFTWARE WORK? Software is the computerised instructions that operate the computer, execute particular functions or tasks, and manipulate the data. For software (the instructions) to perform various functions, it must be programmed. That is, the instructions need to be written in a programming language that the computer can understand. Without a program, a computer is useless. Programming language: An artificial set of rules, vocabulary and syntax used to instruct the computer to execute certain tasks. Computer program: A sequence of instructions that can be executed by a computer to carry out a process. Over the years, a wide range of programming languages have been developed, including BASIC, FORTRAN, PASCAL, C++, JAVA, and so on. Each language has a unique set of words (codes) that it understands and a special syntax for organising program instructions. The language the computer actually understands is called machine language, which comprises numbers only. This language is used by the computer to understand the programming language and translate the terms into executable instructions. Lying between programming languages and machine languages are assembly languages. Assembly languages have the same structure and set of commands as machine languages but they enable a program to use names instead of numbers. UNDERSTANDING COMPUTERS 11 Please note: most people who use computers today do not need to worry about programming, machine, or assembly languages. This is because the software being used today is written in a highly user-friendly manner and in a way that does not require knowledge of the computer languages which were used to create and use it. User friendly: Computer software or hardware that is simple to set up, run and use. There are two kinds of software, systems software and applications software. Systems software includes the operating system and all the utilities that enable the computer to function. The most important program that runs on a computer is the operating system. Every general-purpose computer must have an operating system in order to run other programs. This includes controlling functions such as the coordination of the hardware and applications software, allocating storage facilities, controlling the input and output devices and managing time sharing for linked or networked computers. In many respects an operating system works like an air traffic controller to coordinate activities within the computer. Examples of operating systems are Windows NT, DOS and OS/2. The Windows family of operating systems includes a Graphical User Interface (GUI) that makes the software user friendly. Operating system: A collection of software that allows a computer to function. Applications software includes programs that users access to carry out work. They include applications for the following functions. • Word processing is the most common applications software. The great advantage of word processing over using a typewriter is that you can make changes without retyping the entire document. Word processors make it easy to manipulate and format documents. • Spreadsheets are computer programs that let people electronically create and manipulate spreadsheets (tables of values arranged in rows and columns with predefined relationships to each other). Spreadsheets are used for mathematical calculations such as accounts, budgets, statistics and so on. • Database management applications are computer programs that let people create and manipulate data in a database. A database is a collection of related information that can be manipulated and used to sort information, conduct statistical analyses or generate reports. • Presentation packages and graphics are computer programs that enable users to create highly stylised images for slide presentations and reports. They can also be used to produce various types of charts and graphs. Many software applications include graphics components including: paint programs, desktop publishing applications and so on. UNDERSTANDING COMPUTERS 12 • Communications applications typically include software to enable people to send faxes and emails and dial into other computers. Software programs are continually being written or upgraded to undertake certain tasks. As a result the software can become obsolete quickly. Software can be either ‘proprietary’ (also called ‘closed’) or ‘open’. Proprietary software is privately owned and controlled. A proprietary software design or technique is one that is owned by a company, which will usually not divulge specifications that would allow other companies to duplicate the product. Increasingly, proprietary software is seen as a disadvantage in many organisations; users prefer to use ‘open’ software, which is software designed using specifications that are publicly accessible. The great advantage of open software is that anyone can create add-on products for it because they can understand how it was designed. People using open software can mix and match products from different manufacturers. By making software public, however, a manufacturer allows others to duplicate its product, which many manufacturers do not want. Activity 4 Choose a computer and switch it on. What operating system does it use? What version? What software applications are being run on the computer? For help, look at the computer and software manuals and have a friend or colleague work with you. HOW DOES A COMPUTER PROCESS INFORMATION? When data is input into a computer, the numbers or words we understand are translated into a binary numbers system. Binary is the language of computers. Everything you type, input, output, send, retrieve, draw and so on is, in the end, converted to the computer’s native language: binary. Binary number system: A numerical system wherein each digit stands for a power of two. The binary system uses only two symbols, 0 and 1, to represent values. UNDERSTANDING COMPUTERS 13 In the decimal system, commonly used in most countries, each digit represents a value of 10. For example, the number 103 would break down as follows: 1 x 100 = 100 0 x 10 = 0 3 x 1 = 3 103 = 103 In the binary system, each digit position represents a value of 2. Because computers use the binary system, powers of 2 play an important role. This is why everything in rd th computers seems to come in 8s (2 to the 3 power), 64s (2 to the 6 power), 128s (2 th th to the 7 power), and 256s (2 to the 8 power). Therefore, in the binary system, the number 103 would break down as follows: 1 x 64 = 64 1 x 32 = 32 0 x 16 = 0 0 x 8 = 0 1 x 4 = 4 1 x 2 = 2 1 x 1 = 1 1100111 = 103 The values in a binary system the 0s and 1s are called ‘binary digits’ or bits. Binary digit (bit): A digit within the binary number system. A bit is the smallest unit of information held in a computer. The computer’s electronic circuits have only two states, on or off, and therefore can only understand 0s and 1s, which may represent such opposites as on or off, yes or no, or up or down. This is why all computers use the binary system. In order to make the bits useful, they are combined into ‘bytes’ of information. UNDERSTANDING COMPUTERS 14 Byte: A combination of bits that represent one character. A byte is usually composed of 8 bits. Computer programmers have developed codes for various bytes of information, so that they may be read by different computer programs. For example, one code might define the letter A as ‘11000001’ and the letter B as 11000010’. The number 6 might be defined as ‘11110110’ and the number 8 as ‘11111000’. When the person entering data strikes the A key on the keyboard, the computer registers it as ‘11000001’. When he or she enters the B, the computer reads it as ‘11000010’. Similarly, the number 6 is understood by the computer as ‘11110110’ and the number 8 as ‘1111100’. In this way, the computer can store words and numbers as binary digits and then retrieve them and convert them back into words or numbers as required. As discussed earlier, this work of manipulating, storing and processing the data takes place in the Central Processing Unit, the computer’s main memory. The CPU consists of an arithmetic and logic unit, or ALU, a control unit, and a set of registers. • The arithmetic and logic unit is the portion of the CPU where arithmetic and logical operations take place. • The control unit is the part of the CPU that supervises the general operations of the computer. • The registers are devices that hold data inside the computer’s memory long enough to execute a particular function, such as indexing, calculating, sorting or otherwise manipulating data. They are the CPU’s own internal memory. Data travels from one part of the computer to another through a kind of path known as a bus. Bus: The channel or path that lets the parts of a computer communicate with each other. Similar to a school bus for school children, a computer data bus picks up a load of data from one of the components on the main computer board and then transfers the data to another component on the main computer board. The main circuit board of a microcomputer is also known as the motherboard. The motherboard is the principal board that has connectors for attaching devices to the bus. Typically, it contains the CPU, memory and basic controllers for the system. The data bus is really a series of electrical circuits that connect the various electrical elements on the main board. The data are input into the computer and processed in the CPU. They travel along the bus to be stored in the computer’s memory. The amount of memory available is described in bytes of information, referring to the combination of bits representing characters. The higher the number of bytes the more memory the computer has. Today’s computers hold ‘megabytes’ or even ‘gigabytes’ of data. A megabyte is a unit of one million bytes; a gigabyte is one billion bytes, and a terabyte is one trillion bytes. If a computer has a memory of 64 megabytes, then it can hold 64 million bytes of information. UNDERSTANDING COMPUTERS 15 Data can be stored so that it is readable again only using the software with which it was created, or it can be stored in other formats, so that it may be transferred or used by other software programs. There is a standard character code used to store data so that it may be used by other software programs; this code is called ASCII or American Standard Code for Information Interchange. The ASCII code assigns a specific pattern of bits to each character, as described above. Another code that may be found, especially in IBM-brand mainframe computers, is EBCDIC, or Extended Binary Coded Decimal Interchange Code. The important point to remember about these codes is that their main value is to store information so that it is readable by other computers. By using ASCII or EBCDIC, it is possible for people to retrieve and use someone else’s data using a different type of hardware or software. The main disadvantage of using ASCII or EBCDIC is that the formatting or other special qualities of computerised information may be lost. WHAT IS COMPUTER MEMORY? As defined earlier, memory refers to the temporary internal storage areas within a computer. The term memory is usually used as shorthand for ‘physical memory’, which refers to the actual chips capable of holding data. Some computers also use ‘virtual memory’, which expands physical memory onto a hard drive. The main type of memory and the most familiar to users is random access memory (RAM). RAM is the same as main memory. A computer can both write data into RAM and read data from RAM. Every time a user turns on his or her computer, a set of operating instructions is copied from the hard disk into RAM. These instructions, which help control basic computer functions, remain in RAM until the computer is turned off. Most RAM is volatile, which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM disappears. The contents of RAM are necessary for the computer to process data. The results of the processing are kept temporarily in RAM until they are needed again or until they are saved onto the hard disk or other storage device. Today the storage capacity of RAM is measured in megabytes (MB). PCs (microcomputers) typically have between 16 and 64 MB of RAM, which means they can hold between 16 and 64 million bytes of data (a standard A4 page of text typically holds about 2,000 bytes or characters of text). Other types of memory include • ROM (read only memory): unlike RAM, ROM is non-volatile and only permits the user to read data. Computers almost always contain a small amount of read-only memory that holds instructions for starting up the computer. UNDERSTANDING COMPUTERS 16 • PROM (programmable read-only memory): a PROM is a memory chip on which you can store a program. Once the PROM has been used, you cannot wipe it clean and use it to store something else. Like ROMs, PROMs are non-volatile. • EPROM (erasable programmable read-only memory): an EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light. • EEPROM (electrically erasable programmable read-only memory): an EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge. Activity 5 Ask the person or persons responsible for computer management in your organisation how much random access memory (RAM) your organisation’s computer has. Find out if they think the computer has enough RAM and why or why not. HOW ARE DATA STORED? It is very important to distinguish between memory, which refers to the circuitry that has a direct link to the computer processor, and the storage, which refers to media such as disks that are not directly linked to the processor. Remember, a storage device is anything that is used to store computer data. Floppy disks, hard disks, optical disks, CDs and magnetic tapes are all types of storage device. Physical storage refers to how data are actually kept on the storage disk. The most commonly used medium for storage is magnetic storage. With magnetic storage the computer stores data on disks and tape by magnetising selected particles of an oxide-based surface coating. The particles retain their magnetic orientation until that orientation is changed. Thus magnetic disks and tapes are modifiable storage media. The two most popular types of magnetic storage media are hard disks and diskettes. Magnetic tape provides a third type of magnetic storage, and optical disk is a new storage medium. Following is a discussion of each; the definitions included earlier are repeated. Hard drive or hard disk storage provides faster access to files than a diskette. Hard drive: The storage area within the computer itself, where megabytes of space are available to store bits of information. Also known as a hard disk. A hard disk platter is a flat, rigid disk made of aluminium or glass and coated with a magnetic oxide. A hard disk consists of one or more platters and their read-write heads. A read-write head is the device that reads the data from the disk platter into the computer. It also records (or ‘writes’) data onto the platters. Hard disk platters in UNDERSTANDING COMPUTERS 17 microcomputers are typically 3½ inches (about 10 centimetres) in diameter: the same size as the circular mylar disk in a diskette. However, the storage capacity of a hard disk far exceeds that of a floppy disk. Also, the access time of a hard disk is significantly faster than a diskette. Unlike diskettes, which begin to rotate when one requests data, hard disks are continually in motion, so there is no delay as the disk spins up to speed. Like diskettes, hard disks provide random access to files by positioning the read-write head over the sector that contains the requested data. Diskette. A small, removable, flexible mylar plastic disk covered with a thin layer of a magnetisable substance, onto which digital data can be recorded and stored. Also known as a floppy disk. Diskettes get another name – floppy disk – from the thin mylar disk. If one cuts open the disk casing (not recommended because the disk will be ruined) one would see that the mylar disk inside is thin and “floppy”. Diskettes are also called floppies. Although today’s microcomputers typically use 3½ inch (about 10 centimetre) disks, you may still find 5¼ inch (about 15 centimetre) disks that were popular many years ago. Finding a disk drive to read 5¼ inch disks may be very difficult. Diskettes are generally used for transporting or shipping data files or for making duplicate copies of data files for back-up purposes. The storage capacity of a diskette varies but is considerably smaller than that of a hard drive. Since the 1960s, magnetic tape has been a popular form of mainframe computer storage. Magnetic tape: A continuous plastic strip covered with magnetic oxide; the tape is divided into parallel tracks onto which data may be recorded by selectively magnetising parts of the surface, or spots, in each of the tracks. The data can then be stored and reused. When IBM introduced its first microcomputer in 1981, the legacy of tape storage continued in the form of a cassette tape drive, similar to those used for audio recording and playback. Using tape as a primary storage device, however, instead of a hard disk is slow and inconvenient because tape requires sequential access rather than random access. Sequential access means that data is stored and read as a sequence of bytes along the length of the tape. To find a file stored on a microcomputer tape storage device, one has to advance the tape to the appropriate location of the file, then wait for the computer to slowly read each byte until it finds the beginning of the file. Like an audio cassette, for example, a user must go through the tape in sequence to find the part he or she wants. Microcomputer users quickly abandoned tape storage for the convenience and speed of random access disk drives. Recently, however, tape storage for microcomputers has experienced a revival, not as a principal storage device but for making backup copies of the data stored on hard disks. The data on magnetic storage can be easily UNDERSTANDING COMPUTERS 18 destroyed, erased, or otherwise lost. Protecting the data on the hard disk is of particular concern to users because it contains so much data, which could be difficult and time consuming to reconstruct. This is why backups are so important. Back up: To copy a computer file or collection of files to a second medium, usually on a diskette or magnetic tape, so that the data are safe in case the original file is damaged or lost. Backups are usually copied to storage devices that can be removed from the computer and kept separately from the original. A tape backup is a copy of the data from a hard disk, stored on magnetic tape and used to recover lost data. A tape backup is relatively inexpensive and can rescue an organisation from the overwhelming task of trying to reconstruct lost data. Backing up electronic data is critical to protecting it from loss or damage. For more information on backing up data and protecting information, see Automating Records Services and Emergency Planning for Records and Archives Services. The most popular types of tape drives for microcomputers also use tape cartridges, but there are several tape specifications and cartridge sizes. A tape cartridge is a removable magnetic tape module similar to a sound or video cassette tape. Quarter inch tape, called QIC, is a tape cartridge that contains ¼ inch (approximately ½ centimetre) wide tape. Depending on tape length, QIC tape capacities range from 340 MB to 2 gigabytes. Digital audio tape, called DAT, was originally an audio recording format, but is now also used for data storage. The 4mm wide DAT tape format storage capacity ranges from 2 gigabytes to 12 gigabytes. In addition to magnetic storage, there is also optical storage. Optical disk: A storage device that uses reflecting surfaces and laser technology to read and write data on a disk. Also known as a laser disk. With optical storage, data is burned into the storage medium using beams of laser light. The burns form patterns of small pits in the disk surface to represent data. The pits on optical media are permanent, so the data cannot be changed. Optical media are very durable, but they do not provide the flexibility of magnetic media for changing the data once they are stored. UNDERSTANDING COMPUTERS 19 There are three types of optical disks. • CD-ROM’s are the most popular type of optical storage. CD-ROM stands for Compact Disc Read Only Memory. A computer CD-ROM disk, like its audio counterpart, contains data that has been stamped on the disk surface as a series of pits. To read the data on a CD-ROM an optical read head distinguishes the patterns of pits that represent bytes. CD-ROM disks provide tremendous storage capacity. CD-ROMs usually come with data already written onto them. These days most applications software is provided on CD-ROM. • It is now possible for computer users to write data to an optical disk. These are known as WORM disks, which stands for ‘Write Once Read Many’. A single CD holds up to 680 megabytes, equivalent to over 300,000 pages of text in character format, and these disks are quite durable. These CDs are know as CD re-recordable (CD-R). There are other types of WORM disks, although there is no standard for these. • There is a third type of optical disk which can be erased and use to rewrite new information. These are sometimes known as EO (erasable optical) disks and CD-RW (CD rewritable). Magneto-optical disks combine magnetic disk and CD-ROM technologies. Like magnetic media they can read and written to and like floppy disks they are removable. They can store over 200 MB of data, and speed of access to this data is faster than a floppy but slower than a hard drive. There is no universal standard for these yet. CD-ROMs and magnet-optical disks are very useful for storing images. These take up much more storage space than data in character format, such as in word-processed files. Activity 6 Are there procedures for backing up data in your organisation? If so, how often are backups done? What are the data stored on? If the computer stopped working, would the organisation be able to retrieve the data? THE IMPORTANCE OF DOCUMENTATION The computer hardware, software and peripherals will be accompanied by documentation, which explains how the various systems or programs operate. Documentation: Information needed to develop, use or maintain computer hardware and software and to permit access and retrieval of the data. UNDERSTANDING COMPUTERS 20 Although documentation usually comes in the form of printed manuals, guide books, it may also come in electronic form, such as in ‘help screens’ contained within the computer software or documentation in data dictionaries and so on. Often, documentation is created when a specific computer application is used. For example, if a government office creates a data base of employees in order to administer payroll, documentation may be created describing how the database was established, what it is to be used for and what data fields have been created. This documentation can be critical to understanding the database system, particularly if it is being managed in an archival environment long after the creators of the system have left. Documentation should always be retained. Disposing of documentation can cause problems in the future, particularly if you have to reload software from the beginning. Documentation can also contain licences for the legitimate use of the software. Therefore it is critical to protect documentation and ensure it is readily available. PROTECTING AGAINST VIRUSES Virus: A computer program that is planted in one computer and then transferred, hidden in useful information, to one or more other computers with the intention of corrupting or wiping out information in the recipient computer. Viruses are extremely common. Many different types of viruses exist, ranging from ones that prevent you from opening up word-processed documents through to ones that destroy the entire contents of a hard drive. Viruses can be caught in several ways, for example by exchanging floppy disks, via electronic mail messages and through downloading documents from the Web. Networks make it much easier for viruses to move around. Once a virus gets into an organisation’s computer system it can spread very quickly. New viruses appear each week, so it is essential that users have up-to- date anti-virus software to combat this problem. If users have software that is unlicenced it could mean that it is ‘pirate’ or ‘bootleg’ software. This is software that has been illegally copied. Software of this nature sometimes contains viruses that can be extremely damaging to computers. UNDERSTANDING COMPUTERS 21 SUMMARY This lesson introduced basic information about how computers work, including information about • the components of a computer • how a computer works • how software works • how the computer’s memory works • how data is stored. UNDERSTANDING COMPUTERS 22 STUDY QUESTIONS 1. In your own words, explain what a computer is. 2. Describe four different types of computers. 3. Briefly explain how a computer works. 4. What is hardware? 5. What is software? 6. What is the central processing unit of a computer? Why is it so important to the operation of the computer? 7. Describe at least four different applications, or uses, of computers. 8. What is a binary number system? 9. What is a bit? 10. What is a byte? 11. What is the meaning of computer ‘memory’? 12. What is RAM? 13. What is a storage device? 14. What is a diskette? 15. What is a hard drive? 16. What is a database? 17. What are the key differences between a diskette and a hard drive? 18. What is magnetic tape and how is it different from a hard drive? 19. Explain the concept of backing up files. UNDERSTANDING COMPUTERS 23